1. Field of the Invention
The present invention relates to a two-component start developer having toner and a carrier mixed with each other in a predetermined ratio which is used for an image forming apparatus utilizing a so-called electrophotographic method such as an electrostatic copying machine or a laser beam printer and a method of controlling the toner density in forming images usint the start developer.
2. Description of the Prior Art
In this electrophotographic method, a photoreceptor is first exposed to form an electrostatic latent image on its surface. A developer containing toner is then brought into contact with this electrostatic latent image, to develop this electrostatic latent image into a toner image. This toner image is transferred to the surfaces of paper sheets from the surface of the photoreceptor and is fixed to the surfaces of the paper sheets by, for example, applying pressure and heat.
As the developer used in the above described electrophotographic method, a two-component developer containing toner and a carrier is generally used. The carrier is made of a magnetic material such as ferrite powder and circulates in a developing device for developing an electrostatic latent image into a toner image with the toner being electrostatically adsorbed thereon.
A developer first used in newly manufacturing an image forming apparatus utilizing the above described electrophotographic method or after maintenance such as repair and a check of the image forming apparatus is one having toner and a carrier mixed with each other in a predetermined ratio according to the conditions such as the image densities of images, which is referred to as a start developer.
In the start developer, the image density of an image in the early stage of image formation and the amount of charge based on, for example, the presence or absence of occurrence of scattering of toner at the time of image formation are defined in addition to the above described mixing ratio of the toner to the carrier.
However, there have been conventionally problems irrespective of the above described definition that the image formed is fogged, the toner is scattered in the image formed and the image forming apparatus, and the resolution of the image formed is decreased in addition to the problem that the image density is significantly lowered as shown in FIG. 8 from the early stage of image formation to a time period during which image characteristics are stabilized (referred to as "stable time period" hereinafter) after repeating image formation approximately 3000 times.
Furthermore, when the same type of start developers are used for a plurality of image forming apparatuses, there is a problem that the degree of occurrence of defects such as fogging varies in addition to the problem that the image density varies for each image forming apparatus as image formation is repeated.
The inventors of the present application have found the following cause and effect relation between the start developer and inferior images as a result of examining the causes of occurrence of various defects from the early stage of image formation to the stable time period from various viewpoints.
More specifically, the toner density (T/D %) of the developer and an output voltage (V) of a magnetic sensor in measuring the permeability of the developer by the magnetic sensor are in the relation represented by a solid line in a graph of FIG. 7(a) (referred to as T/D-V characteristics hereinafter). In the conventional image forming apparatus, therefore, the permeability of the developer is measured by the sensor and the toner density is estimated from a carve of the T/D-V characteristics to control the supply of toner. That is, in this image forming apparatus, operations are programmed so as to judge that the toner density of the developer is below a predetermined value when the output voltage of the sensor exceeds a threshold value V.sub.T at which the supply of toner is started to automatically supply toner.
Meanwhile, the above threshold value V.sub.T corresponds to the output voltage of the sensor in a case where the toner density of a developer in the stable time period is D.sub.b, as shown in FIG. 7(a). When the developer having a toner density of D.sub.b in the stable time period is used, it is found that an image having an image density I.sub.T is obtained from the relation between the toner density (T/D %) of the developer and the image density (ID) of an image transferred to a paper sheet (referred to as T/D-ID characteristics hereinafter) which is represented by a solid line in a graph of FIG. 7(b).
In a conventional start developer, however, the output voltage of the sensor is slightly higher or lower than that in the developer in the stable time period, so that some shift may occur between the toner density analogized from the T/D-V characteristic curve and the actual toner density. Particularly when a start developer is used in which the output voltage of the sensor is lower than that in a developer in the stable time period and the T/D-V characteristic curve is shifted on a lower voltage side from the T/D-V characteristic curve in the developer in the staple time period (represented by the solid line in FIG. 7(a)), as represented by a two-dot and dash line in FIG. 7(a), the above described defects such as lack of image density, fogging, scattering of toner and decrease in resolution are liable to occur.
The foregoing will be described in more detail.
More specifically, consider a case where the start developer in which the T/D-V characteristic curve is shifted on the lower voltage side from the T/D-V characteristic curve in the developer in the stable time period is used as described above. In this case, if toner is consumed, the toner density of the developer is gradually decreased from D.sub.a which is its initial value and correspondingly, the output voltage of the sensor is gradually increased from V.sub.S which is its initial value along the T/D-V characteristic curve represented by the two-dot and dash line. When image formation is repeated approximately 100 times, the output voltage of the sensor reaches the above described threshold value V.sub.T at which the supply of toner is started.
However, the actual toner density of the developer in which the output voltage of the sensor reaches the threshold value V.sub.T is decreased to D.sub.c which is lower than the toner density D.sub.b in the developer in the stable time period because there is some shift between the T/D-V characteristic curve (represented by the two-dot and dash line) in the start developer and the T/D-V characteristic curve (represented by the solid line) in the developer in the stable time period.
Moreover, in this start developer, a curve of T/D-ID characteristics is also shifted to the side of a lower image density (on the lower side in FIG. 7(b)) from the T/D-ID characteristic curve (represented by the solid line in FIG. 7(b)) in the developer in the stable time period, as represented by a two-dot dash line in FIG. 7(b).
Consequently, the image density significantly drops, as indicated by an arrow represented by a two-dot and dash line in FIG. 7(b), resulting in lack of image density.
Furthermore, the toner density of the start developer has been conventionally set to a higher value D.sub.a such that an image having a predetermined image density (I.sub.T as described above) can be obtained at the time of starting the use of the developer, as represented by the two-dot and dash lines in FIGS. 7(a) and 7(b). Consequently, excessive toner exists in the developer in the early stage of image formation, as compared with the developer having a toner density of D.sub.b at which an image having the same image density I.sub.T can be obtained in the stable time period. Consequently, fogging, scattering of toner and the like occur and the resolution is decreased due to the excessive toner.
After the output voltage of the sensor reaches the above described threshold value V.sub.T, the following pattern is repeated. More specifically, toner is supplied when the output voltage slightly exceeds the threshold value V.sub.T. After the toner is supplied, image formation is repeated. Consequently, the output voltage slightly exceeds the threshold value V.sub.T, so that toner is supplied again. In addition, when image formation is repeated as described above, the T/D-V characteristics in the developer gradually approach the solid line from the two-dot and dash line, in FIG. 7(a).
In this stage, therefore, the output voltage of the sensor and the toner density are shifted, as represented by a zigzag line in FIG. 7(a). Correspondingly, the toner density and the image density are shifted, as represented by a zigzag line in FIG. 7(b), to gradually increase the image density. However, image formation must be repeated approximately 3000 times as described above to a time period during which the T/D-V characteristic curve in the developer coincides with the T/D-V characteristic curve in the developer in the stable time period which is represented by the solid line and the T/D-ID characteristic curve in the developer coincides with the T/D-ID characteristic curve in the developer in the stable time period, that is, the stable time period. Accordingly, during that repetition, the defects such as lack of image density shown in FIG. 8 and fogging continuously occur.
Furthermore, in the shift stage represented by the zigzag lines, a phenomenon occurs that the output voltage of the sensor is not changed irrespective of the gradual increase in the actual toner density. Accordingly, the supply of toner becomes excessive. As a result, occurrence of the defects such as fogging is promoted.
The reason why the image density varies and the degree of occurrence of defects such as fogging varies when the same type of developers are used for a plurality of image forming apparatuses is that there is a variation in characteristics between sensors therein.
More specifically, if there is a variation in characteristics between the sensors, there arises a difference between output voltages of the sensors when developers having the same permeability are measured. Consequently, the above described T/D-V characteristic curve is shifted up and down for each sensor and for each image forming apparatus, as shown in FIG. 9.
In the conventional image forming apparatuses, however, the threshold values V.sub.T have been always set to a constant value irrespective of the above described variation in characteristics between the sensors. Therefore, even if the developers having the same properties are used for a plurality of image forming apparatuses, the actual toner density of the developer in which the output voltage of the sensor reaches the threshold value V.sub.T is shifted for each image forming apparatus. As a result, the image density varies and the degree of occurrence of defects such as fogging varies for each image forming apparatus.